1. Field of the Invention
This invention relates to acoustic ground vibration detectors which are used to detect vibrations in the ground and render them audible. Such ground vibration detectors are used inter alia in disaster control and rescue work, especially for detecting knocking sounds from persons buried under the surface and locating the trapped persons.
2. Description of the Prior Art
Known ground vibration detectors operate with one or more three-component geophone probes. These three-component geophone probes contain three individual geophones operating on electrodynamic principles to detect vibrations in the ground. The effective axes of vibration of the individual geophones are positioned like the three axes x, y, z of a three-dimensional, rectangular system of coordinates;see e.g. the leatlet "3D unit", July 1984 of the Dutch company Sensor Nederland bv, 2251 AP Voorschoten. This arrangement enables the three-component geophone probe to detect P-waves (primary waves or longitudinal waves), S-waves (shear waves or transverse waves) and O-waves (surface waves). These different types of waves are propagated at different velocities; the high frequency primary wave has the highest propagation velocity, the low frequency shear wave a medium velocity and the surface wave is propagated at the lowest velocity.
The geophone probes of the known ground vibration detectors are connected by a cable to an amplifier arrangement by which the signals corresponding to the ground vibrations received by the individual geophones are rendered audible, e.g. in hoadphones, and processed to locate the source of the ground vibration.
The known ground vibration detectors are, however, not entirely satisfactory as they are only able to process frequencies within the audible range, i.e. above 30 Hz or thereabouts. In some cases, it is necessary to detect ground waves at very low frequencies far below the range to which the human ear is receptive. Thus it has been found, for example, that in areas of soft ground or compacted sand or areas covered with grass or rubble or loose stone, ground vibrations at audible frequencies, say above 30 Hz, may be entirely absent, at least if these areas are at some distance from the source of the ground vibration, e.g. from a person trapped underground, but in such cases these areas still carry ground vibrations at a very low frequency which could be described as "ground infrasound".